This invention relates to shortening the period of time a tire spends in a curing press, and more particularly to the last portion of this period which is referred to as the blowdown period.
The simple fact that the cost of producing a tire can be reduced by reducing the period of time it takes to vulcanize ("cure")a tire has sensitized major tire manufacturers to the criticality of saving a fraction of a minute in the curing cycle. Over the years, the effort to minimize curing time without sacrificing quality, has been unremitting. This effort has included using higher curing temperatures, changes in rubber compounding, mechanical improvements in the curing presses, improvements in the valving, and switching from lower pressure steam to higher pressure steam. Details about such systems are referred to in U.S. Pat. Nos. 2,066,265; 3,489,833; 3,579,626; 4,027,543; 4,126,657; 4,371,843; and, 4,490,325.
Referring particularly to the last ('325) patent, there is disclosed a multistage curing process in which a hot water (second stage) cure is sandwiched between first and third stage steam cures. Blowdown of the steam from the third stage saves about half the time the blowdown requires if the hot water cure was the last stage. This is because it is quicker to blowdown steam than it is to blowdown steam and remnants of water. It is the saving of a few seconds of this `quicker` time with which my invention is now concerned.
A typical cycle for a passenger tire is about 15 min. For economic reasons, it is desirable to shrink the period for vulcanizing the tire to the minimum. Saving 30 secs in the curing cycle of a passenger tire may add up over a period of time, to a several million dollar advantage for the manufacturers, provided of course, this can be done without sacrificing the cured quality of the tire produced. To this end, the '325 patent sought to improve the efficiency of the latter portion of the curing cycle where the main curing occurs, as did the teaching of U.S. Pat. No. 3,579,626; and the invention of U.S. Pat. No. 3,489,833 sought to improve the efficiency of warming up the tire in the initial portion of the curing cycle. Other teachings have focussed one or both of the aforesaid portions of the curing cycle. None, to my knowledge, is devoted to reducing the blowdown time.
By "shaping" we refer to the initial inflation of the bladder to permit it to press against the inner surfaces of the tire uniformly and thus displace air between the bladder and the tire. Shaping may be done in several steps. For example, the pressures may be sequentially increased from 2 psig to 5 psig, then to 10 psig, and held for a different interval of time at each pressure level. The internal pressure during the curing step may likewise be "stepped" and held at each pressure level for a different interval of time.
Those skilled in the art of curing tires will recognize that a tire is fully cured to desired "cured specifications" when the blowdown portion of the curing cycle is commenced, though some additional curing will continue while the tire is being blowndown, and even after it is removed from the curing press. But such additional curing, or over-curing, is not part of the essential curing covered by or within the cured specifications. Over-curing is generally undesirable, though some over-curing at the outer portions of a tire is to be expected when adequate curing is provided at the point of least cure. Thus, it seemed desirable to shorten the blowdown period if only to minimize over-curing a tire.
The blowdown portion of the cycle is the time it takes to purge steam from the bladder in each mold cavity until the pressure is sufficiently low to open the mold. Depending on numerous factors, some of which are detailed hereinafter, this period is fixed by a timer for each press and is about 30 sec for a passenger tire. If, for some reason, the pressure in the bladder is not low enough, the press will not open because of a pressure safety switch which mechanically responds to pressure on a diaphragm. The safety switch is set for a pressure low enough so as not to injure an operator who happens to be close by when the press opens. Such pressure is in the range from about 3 psig to about 10 psig.
Each press is already equipped with a mechanical pressure safety switch, typically a diaphragm type. A similar mechanical pressure switch may be used to trigger the ring drop (for radial tires) when the pressure reaches a predetermined level, say 2 psig. Still another mechanical pressure switch, also of the diaphragm type, is used to warm up and pulse a newly installed bladder, or start up a cold press. With these mechanical pressure sensors and their inherent advantage in coping with a power failure, there is no reason to use an electronic pressure sensor. Moreover, since the blowdown sequence and all other curing operations in conventional presses are timer controlled, there is no reason to desire the installation of yet another pressure sensor, whether mechanical or not. Most of all, trying to monitor pressure sensitively enough to affect the blowdown period seemed unlikely to be rewarding.
Thus, trying to save a few seconds, in the range from 3 to 15 secs, in the already brief blowdown period seemed, at first, to be misdirected effort because it was directed to what already was the shortest portion of the curing cycle; and, because the instrumentation to control such a fine adjustment in cycle time would be too sophisticated and demanding for reliable operation in a curing room of a tire plant.
The curing room typically houses from 50 to about 250 curing presses which are fed by central steam and water systems through steam and water manifolds to which each press is connected with appropriate valving. Of particular concern to this invention is the blowdown manifold to which each press is connected. This manifold is typically a relatively large diameter steam line, in the range from 8" to 12" diam. depending upon the number of presses commonly manifolded; or, a relatively large diameter water blowback line, in the range from 10" to 14" diam. for presses in which hot water is recovered for reuse. Steam, used to displace the trapped hot water is later blowndown.
The steam blowdown lines from each press to the manifold are also relatively large, in the range from 1" to 2", considering that their function is simply to exhaust the steam trapped in the mold near the end of the curing cycle.
The blowback portion of the cycle is also referred to as the hot water recovery portion of the cycle when hot water is used in the cure.
Each press may include a single, but usually two, simultaneously operated molds and the press is preferably individually controlled by its own programmable controller ("PC") and the necessary instrumentation and hardware which allows a press to be operated automatically. The curing instructions for the PC may be downloaded from a central computer each time the curing cycle is to be changed.
A curing press may be of the `pot-heater` type referred to in U.S. Pat. No. 4,371,483, in which a stack of split-molds loaded with green tires is formed within a pressure vessel closed at the top, with a dome having a butt-plate against which the stack is biased by a hydraulic platform on which the stack rests. Conventional potheaters are of the type manufactured by United McGill of Columbus, Ohio or Pennsylvania Engineering Corp. of Newcastle, Pa., inter alia.
Another curing press is made by McNeil Corp. under the Bag-O-Matic trademark. This press uses an inflatable elastomeric bladder which is raised and lowered on a central shaft axially disposed within a green tire in a mold cavity. Still another popular press is made by NRM Corp. under the Autoform trademark. This latter press is also referred to as the "bag-well" press because it uses a bag (bladder) in a central well, and a ram pushes the bag down before a cured tire is removed.
Regardless of the type of curing presses used in the curing room, each is manifolded to a common blowdown manifold; though each press in a present-day curing room is controlled by a PC, the blowdown period on prior art presses is fixed; that is, a timer is set which establishes when a blowdown period is completed. The length of the period is fixed from experience, and trial and error. Stated differently, it is experimentally established how long it takes for the pressure in the bladder to fall low enough so that the mold can be opened safely, and this period, with additions for expected variations in steam pressure, and for a margin of safety, is the period set on the timer.
The presses are loaded sequentially, so that an operator may successively load each of a plurality of presses as he proceeds down the line of presses in the curing room. However, not all the presses are operated on the same curing cycle, and even if they were, the staggered sequence of their operation may not be preserved from one operating shift to another. Variations in the open/close cycle are generally cumulative, and to be expected. For example, there may be a delay in stripping a cured tire, or the loader may pick up a tire incorrectly and the press must be stopped, or there is a bladder failure, or the operator may simply have run out of green tires for that press. As a result, the blowdowns of several presses may overlap. If these presses are in the same row, or are in different rows but in pressure-sensing relationship with one another through the common manifold, overlapping blowdowns will increase the pressure in the blowdown manifold, and the pressure differential between the bladder and the blowdown manifold will be smaller than when there is no overlap.
Other causes for a change in the pressure differential may be a leak of hot water, or steam, into the blowdown manifold, sufficient to increase the pressure therein; or, a restriction in a blowdown valve, or blowdown line, typically caused by a small piece of rubber, broken off from the bladder.
Since any of the foregoing reasons will cause a smaller pressure differential, therefore will require a longer blowdown than is set on the timer, it is probable that the timer (with a fixed period) on the PC will permit the press to open when the pressure in the bladder is too high. Therefore a safety pressure switch is provided on each press which will not permit it to open.
In most instances, under normal operating conditions, the press is blowndown well within the fixed time period. For example, if the time period set is 30 sec, the press is typically blowndown to less than 5 psig within 20-25 secs so that, in actual normal operation, the press could have opened 5-10 secs earlier.
Since a pressure safety switch is provided on each press, it seemed self-evident that this switch should be used to permit the press to open immediately upon the predetermined pressure having been reached, rather than have the press operate with a fixed blowddown period. For example, one could set the timer for an unrealistically short time, say 10 secs, relying solely on the pressure safety switch to allow the press to open when the presure is low enough. In other words, one would be using the pressure switch on each press to provide a variable blow-down time which would be the shortest interval for the particular conditions extant at the blowdown of that press. Except there would be no safety if the pressure safety switch malfunctioned - a problem easily solved by using two pressure safety switches. Except that pressure switches are electromechanical devices which are prone to drift, therefore require frequent calibration; and they are susceptible to damage from the curing media, and also to mechanical wear and aging. Maintaining a single pressure safety switch is an unenviable task, and adding a second such switch results in exponentially compounding the maintenance problem--which is not quite what plant operating personnel favor.
Since a temperature probe in the blowdown line would essentially instantaneously register the temperature of the steam, it appeared that the temperature corresponding to the desired steam (low) pressure, say 5 psig, could provide the signal to open the press. But a sufficiently sensitive temperature probe is not easily available.
Still another consideration was the use of an electronic strain gauge which was sensitive enough to distinguish the difference in strain between the halves of the mold when the pressure in the bladder was sufficiently low, but the sensitivity of such a gauge was problematical.
Thus it was that I decided to trigger opening of the press with pressure, but not to use a pressure switch, and somehow to generate a signal corresponding to the instantaneous pressure within the bladder, and use the signal to trigger the opening.